This application claims the priority of German application No. 198 07 880.3, filed Feb. 25, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and device for monitoring the tire air pressure of wheels of an automobile, in particular to a method and device, with which at least for some of the automobile's wheels the tire air pressure (tire pressure) is indirectly measured (rather than directly) via a corresponding tire air pressure sensor.
German patent document DE 43 27 492 C1 discloses various processes and devices for the indirect determination of the tire pressure of vehicle wheels. In these processes and devices, the measurement of the tire pressure is most often limited to very specific conditions, in particular situations which are free of the application of propulsive power. Thus, in the tire pressure warning system disclosed in German patent document DE 43 27 492 C1, the tire pressure is indirectly derived from an adjustment of the number of revolutions of the wheel and a corresponding wheel revolutions scaling factor. The number of wheel revolutions is measured only when the torque is below a predetermined torque boundary value and when specific other conditions are fulfilled. Specifically, with this process only such driving conditions in which the vehicle rolls somewhat free of the application of propulsive power and without noticeable cornering effects are addressed. This method may lead to a situation where, on inclines or during prolonged highway motoring at higher speeds, i.e., when propulsive powers are permanently applied, tire pressure monitoring does not occur.
German patent document DE 44 00 913 A1 discloses a method and a device of the type previously mentioned. In this process, the speeds of the wheel's revolutions are measured to determine the loss of air pressure in a tire of a driven wheel, and from this measurement the slip values for the driven wheels are computed. In addition, the respective propulsive powers of the wheels are determined and placed into a relation with the slip values as force-slip test data pairs. From the test data pairs, a compensation curve, preferably in the form of a linear regression curve, is determined. The tire pressure is determined from the slope of the straight line regression, and in particular with an increasing slope of the characteristic curve, a loss of tire pressure is indicated.
It is an object of the invention to provide a method and a device for monitoring and determining the tire pressure of automobile wheels at relatively limited costs (indirectly and possibly continuously), in particular during driving conditions when propulsive forces are applied to the wheels of the vehicle.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which for at least one driven wheel, the slip and the longitudinal force of the tires (also called the tire's axial force) are determined in the form of respective longitudinal force of the tires-slip test data pairs. At least for a selected part of the gathered test data pairs, a compensation curve is determined as well as its offset value with regard to the zero point in the longitudinal force of the tires-slip performance graph. From this offset value, the corresponding wheel radius relationship between at least one driven and one non-driven wheel is deduced, and from this the relative tire air pressure of the wheels is determined.
Because the information necessary to determine the slip and longitudinal force of the tires (for example, via the number of wheel revolutions, engine speed and torque as well as driving speed) is already present in modern automobiles (for example, in an anti-lock braking system (ABS)), this process can be realized with a relatively limited effort. Moreover, with this system, relative tire pressure information between one driven and one non-driven wheel is gathered without utilizing a pressure sensor.
In an embodiment of the method according to the invention, specifically those longitudinal forces of the tires-slip test data pairs which reside in the micro-slip range are examined. In this range, the functional connection between the longitudinal force of the tires and the slip, according to experience, approximates a linear relationship which facilitates the use of a straight line as the compensation curve. The latter can be defined with the help of two test data pairs, or as a straight line regression for more than two test data pairs.
In another embodiment of the method according to the invention, the relative determination of tire pressure occurs for the pair of wheels on the same side of the vehicle, as well as for the wheels situated diagonally opposed from each other, by means of which a higher degree of certainty of determining the tire air pressure is attained.
In yet another embodiment of the method according to the invention, on at least one wheel the absolute tire air pressure is measured directly with an assigned pressure sensor, and is used as a reference value for the relative tire pressure value, which is determined for at least one other wheel. With this arrangement, it is thus possible (for example, by using only one pressure sensor on one wheel) to determine the absolute tire pressure for all the wheels of an automobile.
In still a further embodiment of the method according to the invention, the slope of the compensation curve is also calculated, and from this the existing wheel load is determined.
An embodiment of the device according to the invention is specifically suitable for directly measuring the absolute tire pressure with an assigned pressure sensor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.